Yamazaki M, Nakajima F, Ogasawara A, Moriya H, Majeska R J, Einhorn T A
Department of Orthopaedic Surgery, Chiba University School of Medicine, Japan.
J Bone Joint Surg Br. 1999 May;81(3):508-15. doi: 10.1302/0301-620x.81b3.9398.
The multifunctional adhesion molecule CD44 is a major cell-surface receptor for hyaluronic acid (HUA). Recent data suggest that it may also bind the ubiquitous bone-matrix protein, osteopontin (OPN). Because OPN has been shown to be a potentially important protein in bone remodelling, we investigated the hypothesis that OPN interactions with the CD44 receptor on bone cells participate in the regulation of the healing of fractures. We examined the spatial and temporal patterns of expression of OPN and CD44 in healing fractures of rat femora by in situ hybridisation and immunohistochemistry. We also localised HUA in the fracture callus using biotinylated HUA-binding protein. OPN was expressed in remodelling areas of the hard callus and was found in osteocytes, osteoclasts and osteoprogenitor cells, but not in cuboidal osteoblasts which were otherwise shown to express osteocalcin. The OPN signal in osteocytes was not uniformly distributed, but was restricted to specific regions near sites where OPN mRNA-positive osteoclasts were attached to bone surfaces. In the remodelling callus, intense immunostaining for CD44 was detected in osteocyte lacunae, along canaliculi, and on the basolateral plasma membrane of osteoclasts, but not in the cuboidal osteoblasts. HUA staining was detected in fibrous tissues but little was observed in areas of hard callus where bone remodelling was progressing. Our findings suggest that OPN, rather than HUA, is the major ligand for CD44 on bone cells in the remodelling phase of healing of fractures. They also raise the possibility that such interactions may be involved in the communication of osteocytes with each other and with osteoclasts on bone surfaces. The interactions between CD44 and OPN may have important clinical implications in the repair of skeletal tissues.
多功能黏附分子CD44是透明质酸（HUA）的主要细胞表面受体。最近的数据表明，它也可能结合普遍存在的骨基质蛋白骨桥蛋白（OPN）。由于OPN已被证明是骨重塑中一种潜在的重要蛋白质，我们研究了以下假设：OPN与骨细胞上的CD44受体相互作用参与骨折愈合的调节。我们通过原位杂交和免疫组织化学检查了大鼠股骨愈合骨折中OPN和CD44的表达的时空模式。我们还使用生物素化的HUA结合蛋白在骨折痂中定位HUA。OPN在硬骨痂的重塑区域表达，在骨细胞、破骨细胞和成骨祖细胞中发现，但在其他显示表达骨钙素的立方形成骨细胞中未发现。骨细胞中的OPN信号分布不均匀，而是局限于OPN mRNA阳性破骨细胞附着于骨表面的部位附近的特定区域。在重塑骨痂中，在骨细胞陷窝、沿小管以及破骨细胞的基底外侧质膜上检测到强烈的CD44免疫染色，但在立方形成骨细胞中未检测到。在纤维组织中检测到HUA染色，但在骨重塑正在进行的硬骨痂区域中观察到的很少。我们的研究结果表明，在骨折愈合的重塑阶段，OPN而非HUA是骨细胞上CD44的主要配体。它们还增加了这种相互作用可能参与骨细胞之间以及与骨表面破骨细胞之间通讯的可能性。CD44与OPN之间的相互作用可能在骨骼组织修复中具有重要的临床意义。
